docs: def as_bel(self) -> str:
          """Return this fusion as a BEL string."""
          return '{}(fus({}:{}, "{}", {}:{}, "{}"))'.format(
              self._func,
              self.partner_5p.namespace,
              self.partner_5p._priority_id,
              self.range_5p.as_bel(),
              self.partner_3p.namespace,
              self.partner_3p._priority_id,
              self.range_3p.as_bel(),
          )